1. Field of the Invention
The present invention relates to a current drive circuit and display, and more particularly to a current drive circuit of organic EL elements and display.
2. Description of the Related Art
Since the luminance of emitted light in an organic EL element is determined by the drive current, a current drive can better eliminate variation in the luminance of emitted light than a voltage drive in a display in which a plurality of organic EL elements is arranged in a matrix. Conventionally, a configuration such as shown in FIG. 1 is employed as the current drive circuit of organic EL elements. FIG. 1 is a circuit diagram of a current drive circuit of the prior art. As shown in FIG. 1, the current drive circuit of the prior art is provided with: p-channel MOS transistor M01, p-channel MOS transistor M11, reference current source l1, switch means SW1, and output terminal O1; organic EL element Z1 being connected to output terminal O1 as load. In addition, p-channel MOS transistor M01 and p-channel MOS transistor M11 constitute a current mirror circuit, whereby the current IREF that is generated by reference current source l1 is returned from high level power supply VDD and supplied by way of switch means SW1 to organic EL element Z1 that is connected to output terminal O1. Switch means SW1 is consisted of, for example, a p-channel MOS transistor and is ON/OFF controlled by one-bit graduation data signal D1. When switch means SW1 is turned ON, a prescribed return current of the current drive circuit is supplied as drive current IOUT to organic EL element Z1, whereby organic EL element Z1 is illuminated; and when switch means SW1 is turned OFF, drive current IOUT becomes 0 and organic EL element Z1 is extinguished. A similar configuration that employs a bipolar transistor is disclosed in FIG. 7 of Japanese Patent Laid-Open Publication No. 2001-042827.
However, the current drive circuit of the example of the prior art that is shown in FIG. 1 is a configuration in which switch means SW1 is connected between output terminal O1 and the drain terminal of p-channel MOS transistor M11, which is the output terminal of a current mirror circuit. As a result, when switch means SW1 is in the OFF state, the voltage between node A and node B of switch means SW1 is substantially the voltage difference between voltage VDD on the high level power supply VDD and ground, i.e., the low level power supply. In other words, the voltage difference is at an extremely high level that approaches voltage VDD, and the problem therefore arises that a large surge current is generated such as shown in FIG. 2 when switch means SW1 changes from the OFF state to the ON state. As an additional problem, the use of the basic current mirror circuit in the current drive circuit of the example of the prior art shown in FIG. 1 prevents the acquisition of a highly accurate return current.